In most organizations ECN's are items that affect the fit, form or function of the product. Something that the customer will notice and will affect how they use the product. This includes colour or material changes. Even if the material is replaced with a similar material, the validation testing will probably need to be repeated. Most ECN's require customer approval, or at least buy in.
PCN's are changes to how the product is made. A change in the order of how the product is assembled. Adjustment to the length of cooling time, etc. As long as the change does not affect the fit, form or function of the part, including material properties.
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